It is summoners levels that matters

Exactly, this might be the solution to a lot of the problems the current rankings and collection Power system brings about. It is simply not fun to lose or win against other players just because you have better or worse cards and the outcome of the match is known before it even started. I personally don't mind losing against players who are at a similar summoner level but just have more cards available to choose from.

I'm not sure how rankings and a matching system can be implemented based on summoner levels. It is an interesting concept and I'm sure it can be done one way or another.